Inexpensive and flexible integrated circuits are needed to continue the digital revolution. One approach to making inexpensive and flexible integrated circuits is to fabricate semiconductor devices on flexible polymeric substrates using low temperature processes. Chemical bath deposition (CBD), the aqueous analogue of chemical vapor deposition with the constituent ions dissolved in a water solution, can be used to deposit epitaxial semiconductor layers at low temperatures. CBD has been successfully used as a batch process to deposit many semiconductor compounds, including Ag2S, CdS, CdSe, CoS, Cu2S, PbS, SnS, SnO, MoS2, SbS, TiO2, ZnO, SnSe, CdSnS, CuInSe2, and PbHgS. Batch CBD processes also have been used to make thin film transistors. Gan et al., U.S. Pat. No. 6,225,149, for example, entitled “Method to Fabricate Thin Film Transistors and Circuits,” describes forming thin film transistors by preparing a chemical deposition bath and depositing a semiconductor layer using the chemical bath.
In recent years, metal chalcogenide semiconductors have received much attention for electronic and optoelectronic applications. Typically, metal chalcogenides are deposited as binary or ternary compounds including sulfide, selenide and telluride using a variety of techniques, including electrodeposition, vacuum evaporation, Successive Ionic-Layer Adsorption and Reaction, spray pyrolysis, sputtering, chemical vapor deposition (CVD) and chemical bath deposition CBD. Batch CBD processes already have proved useful for fabricating large area devices, such as high efficiency CuInSe2 and CdTe solar cells. P. K. Nair, M. T. S. Nair, V. M. Garcia, O. L. Arenas, Y. Pena, A. Castillo, I. T. Ayala, O. Gomez-daza, A. Sanchez, J. Campos, H. Hu, R. Suarez, M. E. Rincon. Sol. Energy Mater. Sol. Cells. 1998, 52, 313. O. Savadogo. Sol. Energy Mater. Sol. Cells. 1998, 52, 361.
Though CBD has many advantages, it also has drawbacks. One major drawback is the formation of particles, as well as the unwanted deposition that occurs with such processes, which generates waste and causes device defects. For batch CBD processes, the heat needed for chemical reaction may be supplied from the solution bath to the sample surface. With reference to CdS as an example, this results in both heterogeneous CdS nucleation at the surface as well as homogeneous CdS formation in the bath. Hence, with thermal jacket baths (glass beaker etc.) or water baths, significant CdS deposition also occurs on the walls of the vessels. The bath must be stirred continuously to ensure uniform thermal and chemical mixing and to minimize adhesion of homogeneously nucleated CdS particles to the growing film surface. Moreover, the unequal bath volumes used to form desired CdS films generates substantial waste and also creates device defects. Efforts have been made to reduce the bath-to-surface volume using cover plates. P. K. Nair, V. M. Garcia, O. Gomez-Daza, M. T. S. Nair, Semicond. Sci. Technol. 2001, 16, 855. Batch CBD processes also typically require that the deposited material be subjected to relatively high-temperature annealing processes. These high-temperatures generally are not compatible with substrates, such as polymeric materials, that are used to make flexible electronic devices, such as flexible transistors.
Batch CBD processes are quite useful. However, a low-temperature process that combines large area deposition with high utilization and growth rate for high conversion efficiencies and precise control over particle formation and deposition has not yet been demonstrated.